Thyristor controlled series capacitors (TCSC) are used in power systems as controllable devices connected in series with a power line. The control offered by TCSC is an “impedance” type control used for controlled series compensation.
A prior art TCSC module is shown in FIG. 1. The TCSC module comprises a capacitor 2 connected in series with a power line 4. In parallel with the capacitor 2 there is provided a thyristor valve 6 in series with an inductor 8. The thyristor valve in itself consists of two anti-parallel thyristors 6a, 6b in order to provide bi-directional functionality.
The TCSC module can operate in essentially three different modes. In a so-called bypassed mode, the thyristors are gated for full conduction and the TCSC module is slightly inductive because of the inductor in series with the thyristor valve. In a so-called inserted mode, the capacitor 2 is inserted by turning off the thyristor valve, i.e., blocking all gating signals to the thyristors. When the capacitor is inserted, the thyristors can be gated near the end of each half cycle in a manner that can circulate controlled amount of inductive current through the capacitor, thereby increasing the effective capacitive reactance of the module. This concept is referred to as vernier control.
In order to provide for bypassing of the TCSC module, three disconnectors 7-9 are provided as shown in FIG. 1. The term disconnector is intended in this description to refer to devices normally not capable of interrupting a current. In normal operation, the first and second disconnectors 7 and 8 are closed and disconnector 9 is open. In these positions, the disconnectors effectively connect the TCSC module in series with the power line 4.
The TCSC module shown in FIG. 1 can be exposed to short circuit currents during line faults, resulting in stresses on the capacitor. To cope with this situation, a first breaker 10 is provided in parallel with the thyristor valve 6. The term breaker is intended in this description to cover devices capable of breaking or interrupting a current. During short circuit faults, the first breaker 10 is closed in order to ensure that the inductance 8 is included for limitation of capacitor discharge current. The current path in this situation through the breaker 10 and thyristor valve 6 is shown in the figure with a dashed line.
To allow permanent bypass of the module in cases of internal fault or during maintenance, a second breaker 12 is provided in parallel with the capacitor 2. This second breaker, when closed, effectively bypasses the capacitor. The current path in this situation through the breaker 12 is shown in the figure with a continuous line. After the breaker 12 has been closed, the disconnector 9 is closed whereupon the breaker 12 is opened. Then, with all current flowing through disconnector 9 and no current through the TCSC module, the disconnectors 7 and 8 are opened. In this way, the TCSC is effectively disconnected from the power line 4.
Finally, the TCSC module shown in FIG. 1 comprises a varistor 14 connected in parallel with the capacitor 2. The varistor, being a power surge protection device, protects the capacitor in case of a voltage spike appearing across the capacitor.
TCSC modules as many other power line components are subject to cost-reduction demands. Thus, the provision of two different breakers contributes to the overall cost of the TCSC module and is in itself a problem. Also, as the number of components included in the TCSC module increases, so is the probability of internal faults.
A further problem with the configuration of FIG. 1 is the fact that when operating in thyristor bypass mode, the fault current will flow through the inductor 8, resulting in magnetic energy and voltage drop.
A special difficulty arises when restoring normal operation and line current shall be commutated from breaker 12 to breaker 10, since the breaker 10 path includes the inductor 8. In order to be able to open breaker 12 this breaker has to have sufficient breaking capability; a disconnect switch would not be sufficient.